GRP78/BiP alleviates oxLDL-induced hepatotoxicity in familial hypercholesterolemia caused by missense variants of LDLR

Abstract

Abstract Background and Aims- Endoplasmic Reticulum (ER) stress, caused by aberrantly trafficked proteins that are encoded by genetic variants of functional genes, is a critical consequence in human disorders such as familial hypercholesterolemia, cardiovascular and hepatic diseases. In addition to the identification of ER stress as a contributing factor to pathogenicity, extensive studies on the role of oxidized LDL (oxLDL) and its ill effects in expediting cardiovascular diseases and other metabolic comorbidities are well documented. However, our current understanding of its role in hepatic insults is inadequate. Our study elucidates the molecular mechanisms underlying the progression of oxLDL and ER stress-induced cytotoxicity in HepG2. Methods-HepG2 cells stably expressing wild-type (WT) LDLR and missense variants of LDLR that are pathogenically associated with familial hypercholesterolemia were used as the in vitro models. The relative mRNA expression and protein profiles of ER stress sensors, inflammatory and apoptotic markers, together with cytotoxic assays and measurement of mitochondrial membrane potential were carried out in HepG2 cells treated with 100 µg per ml oxLDL for 24 to 48 hours. 1-way or 2-way ANOVA were used for statistical analyses of datasets. Results-ER stress responses are elicited along all three arms of the UPR, with adverse cytotoxic and inflammatory responses in oxLDL-treated conditions. Interestingly, oxLDL-treated ER-stressed HepG2 cells manifested intriguingly low expression of BiP- the master regulator of ER stress, as observed earlier by various researchers in liver biopsies of NASH patients. In our study, we show that overexpression of BiP rescues hepatic cells from cytotoxic and inflammatory mechanisms instigated by ER stress in combination with oxLDL, along the ER and mitochondrial membrane and restores cellular homeostasis. Conclusion-Our data provide interesting leads that identify patients with familial hypercholesterolemia conditions and potentially other ERAD diseases as highly susceptible to developing hepatic insults with molecular signatures similar to those manifested in NAFLD and NASH.